1. Field of the Invention
The present invention relates to novel optically active compounds and liquid crystal compositions comprising said compounds. The present invention provides, in particular, ferroelectric liquid crystal materials, for example, optically active compounds and liquid crystal compositions comprising said compounds both useful as electrooptic switching elements (e.g. liquid crystal display devices) in liquid crystal optical modulators, as well as liquid crystal optical modulators using liquid crystal compositions comprising said compounds.
2. Related Art Statement
Liquid crystal display devices have various excellent features such as low-voltage operability, lower electricity consumption, being thin and light-weight, being a non-emissive type and easy on the eye, etc. Accordingly, they are in wide use as various display devices.
Liquid crystal display devices using a nematic liquid crystal operating in the so-called twisted nematic mode (TN mode) are in use currently. However, display devices using this kind of nematic liquid crystal have the drawback of being very slow in response as compared to luminescent type display devices such as CRT, EL and the like. When the liquid crystal display devices using a nematic liquid crystal are applied in a large-scale display device capable of displaying a large amount of information, it is impossible to obtain a display of good contrast because of insufficient threshold characteristic. Thus the liquid crystal display devices using a nematic liquid crystal have had a limitation for wide application. There has recently been developed a liquid crystal display device using a nematic liquid crystal operating in the so-called super twisted nematic mode (STN mode) or SBE and capable of giving a display of improved contrast because of improved threshold characteristic. Even in this STN mode liquid crystal display device, however, the response is not sufficiently improved, and therefore said device has a limitation for application to displays capable of displaying a still larger amount of information. Hence, various attempts have been made to develop a new liquid crystal display system which is applicable to large-scale displays capable of displaying a large amount of information.
Ferroelectric liquid crystals have a memory characteristic and give a high speed response, and accordingly their application to large-scale displays is highly expected. As liquid crystals having ferroelectric properites, there are known those showing a chiral smectic C phase, a chiral smectic H phase, a chiral smectic J phase, etc. Of these ferroelectric liquid crystals, those showing a chiral smectic C phase are thought to have highest practical utility. Ferroelectric liquid crystals showing a chiral smectic C phase were first synthesized in 1975 by R. B. Meyer et al.; one typical example thereof is 2-methylbutyl 4-(4'-n-decyloxybenzylideneamino)cinnamate (hereinafter abbreviated to DOBAMBC) [J. Physique, 36, L-69 (1975)]. A thin film liquid crystal cell was prepared using DOBAMBC and was found to have a high speed response in the order of .mu.sec and a memory characteristic [N. A. Clark et al., Appl. Phys. Lett., 36, 89 (1980)].
Since that time, there was started the development of optical modulation devices (e.g. liquid crystal display devices, photo-printer heads) using a ferroelectric liquid crystal showing a chiral smectic C phase (hereinafter may be referred to simply as "ferroelectric liquid crystal").
As a result, a number of ferroelectric liquid crystal compounds showing a chiral smectic C phase have been developed since then, and various ferroelectric liquid crystal compounds are already known. However, no ferroelectric liquid crystal compound is found yet which has satisfactory reliability and capability for use in large-scale displays, etc.
In order for a ferroelectric liquid crystal to be practically used in a liquid crystal display device, etc., the liquid crystal must be superior in high speed response, orientation, memory characteristic, characteristic of threshold voltage, temperature dependences of these properties, etc. Also, the ferroelectric liquid crystal is required to show a chiral smectic C phase over a wide temperature change so that it can operate within a sufficiently wide temperature range including room temperature, and further to have excellent physical and chemical stabilities.
Of these requirements, particularly important are physical and chemical stabilities and stable expression of high speed response and memory characteristic.
It is reported by Clarkl et al. based on their experiment that a response in the order of .mu.sec is possible under certain conditions. However, even if the conditions used by Clark et al. could have materialyzed a large-scale display capable of displaying a large amount of information, i.e. a display having a very large number of pixels, the display must show a faster response.
The response time (.tau.) of a ferroelectric liquid crystal is approximately given by the following formula when a torque generated by the dielectric anisotropy and an external electric field is neglected. EQU .tau.=.eta./PsE
(.tau. is a response time, .eta. is a viscosity coefficient, Ps is a spontaneous polarization, and E is an applied electric field). Therefore, increase in spontaneous polarization is effective to obtain a faster (shorter time) response.
Meanwhile, memory characteristic is considered to improve dependently upon the value of spontaneous polarization. Increase in spontaneous polarization gives rise to increase in polarization electric field, which brings about uniformity of dipole moment, i.e. stabilization of memory condition.
Thus, increase in spontaneous polarization is very effective for the simultaneous solution of the two tasks perculiar to ferroelectric liquid crystals. Hence, development of ferroelectric liquid crystal compounds with increased spontaneous polarization has recently been pushed forward. As a result, there have been reported, for example, ferroelectric liquid crystal compounds of ester type using, as an optically active group, (R)- or (S)-1-methyl butanol or (R)- or (S)-1-methylheptanol, which are stable physically and chemically [K. Terashima et al., Mol. Cryst. Liq. Cryst., 141, 237 (1986)]. These compounds have a relatively high spontaneous polarization of 50 nC/cm.sup.2 or more, but the value is not sufficient.
In order to obtain a larger spontaneous polarization, there have been synthesized compounds having two asymmetric carbon atoms in the optically active group which is essential for the expression of a chiral smectic C phase. These compounds include, for example, liquid crystal compounds having a dichiral epoxide side chain [David M. Walba et al., Journal of American Chemical Society, 108, 7424 (1986)], and liquid crystal compounds having a halogen atom and a methyl group on two adjacent asymmetric carbon atoms [cf. e.g. JP-A-168780/1985, 218358/1985, 68449/1986, 30740/1987, 46/1987, 103043/1987, 111950/1987, 142131/1987, 175443/1987].
A typical example of the above liquid crystal compounds is 4'-octylcarbonyloxy-4-biphenyl (S)-3-methyl-2-chloropentanoate [JP-A-68449/1986]. This liquid crystal compound has a very large spontaneous polarization of 180 nC/cm.sup.2, but, being an aliphatic chloro compound, has poor chemical stability. Hence, there has been synthesized 4'-octylarbonyloxy-4-[(S)-2-methoxy-(S)-3-methylpentyloxycarbonyl]biphenyl [JP-A-228036/1987]. This compound has excellent chemical stability but has a small spontaneous polarization of 17 nC/cm.sup.2. Thus, no compound has been developed yet which is chemically stable and yet has a large spontaneous polarization.